linux-kernel - Re: [PATCH] kretprobes: reject registration if a symbol offset is specified

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Date:   Tue, 14 Feb 2017 19:32:21 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kretprobes: reject registration if a symbol offset is
 specified

On Tue, 14 Feb 2017 14:01:18 +0530
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:

> Users shouldn't be able to specify an offset with kretprobes, as we always
> want to probe at function entry. Otherwise, we won't be able to capture
> the proper return address resulting in the kretprobe never firing.
> 

Nack, this should be checked by using kallsyms, since the
many non-exported kernel functions have same name.
Actually perf-probe is trying to put any probes(including return
probe) by using relative address from text-start symbol (_stext
or _text). In this case, kretprobe also can be set by _text+OFFSET.

So please rewrite this by using kallsyms_lookup_size_offset()
which tells you the address is actually on the beginning of
function or not.

Thank you,

> With samples/kprobes/kretprobe_example.c including an offset:
> 	my_kretprobe.kp.offset = 40;
> 
> Before this patch, the probe gets planted but never fires.
> 
> After this patch:
> 	$ sudo insmod samples/kprobes/kretprobe_example.ko
> 	[sudo] password for naveen: 
> 	insmod: ERROR: could not insert module samples/kprobes/kretprobe_example.ko: Operation not permitted
> 
> And dmesg:
> 	[48253.757629] register_kretprobe failed, returned -22
> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
>  kernel/kprobes.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 60a702a05684..83ad7e440417 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1847,6 +1847,9 @@ int register_kretprobe(struct kretprobe *rp)
>  	int i;
>  	void *addr;
>  
> +	if (rp->kp.offset)
> +		return -EINVAL;
> +
>  	if (kretprobe_blacklist_size) {
>  		addr = kprobe_addr(&rp->kp);
>  		if (IS_ERR(addr))
> -- 
> 2.11.0
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>